This invention relates to an optical or photoelectric smoke detector of the Tyndall type having increased efficiency and reliability.
In a Tyndall or light scatter type optical smoke detector smoke particles are admitted into a closed smoke chamber, while outside ambient light is precluded from the chamber. A light beam, projected into a test zone of the chamber, will strike and bounce off of smoke particles in the test zone to produce scattered or diffused light which is then detected by a photo or light responsive sensor to indicate the presence of an alarm condition. In the absence of smoke particles the light responsive sensor is optically shielded from the light source so that little or no stray or reflected light shines on the light sensor, thereby avoiding false alarms. In practice this has been difficult. In the past, various arrangements have been employed for baffling, absorbing and attenuating the light so that the light sensor responds only to scattered or diffused light reflected off of smoke particles and not to any direct light from the source or to any light reflected off of the chamber surfaces. The difficulty of restricting the light received by the sensor to scattered light bouncing off of the smoke particles is substantially increased when the optical smoke detector is located in an environment where undesired dust and dirt are likely to enter the chamber over a period of time and accumulate on the chamber surfaces. Even by making the surfaces black, light from the light source may reflect off of the dust and dirt on the surfaces and be received by the light sensor, causing the sensor to initiate the development of a false alarm signal.
The optical smoke detector of the present invention represents a significant improvement over prior scatter type detectors in that a unique optical arrangement is provided to precisely control the light in the chamber to obtain improved performance and increased efficiency. Moreover, the smoke detector is particularly reliable when subjected to undesired dust and dirt, since the operation of the smoke detector will be effectively immune to the accumulation of dust and dirt within the chamber.